Transparent crystals have been suggested for use as solid etalons for separating stacks of dielectric layers in Fabry-Perot interferometers. See, e.g., A. E. Roche, "Solid Fabry-Perot Etalons as High Resolution Infrared Interferometers", Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 95, (1976), pp. 196-203; A. E. Roche et al., "Performance Analysis for the Cryogenic Etalon Spectrometer on the Upper Atmospheric Research Satellite", Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 364, (1982), pp. 46-58.
For upper atmospheric research involving measurements in the infrared spectral region from about 3.5 microns to 15 microns, crystals of zinc sulfide, zinc selenide, germanium and silicon are sufficiently transparent to function as solid etalons for Fabry-Perot interferometers. Pure silicon crystal is particularly suitable in terms of optical stability and optical throughput for use as a solid etalon in infrared interferometry. Nevertheless, silicon crystal has not heretofore been used as a solid etalon in infrared interferometric applications requiring that the etalon be able to withstand stresses of a rocket launch, and be able to operate in a temperature-cycling environment in which the temperatures range from ambient terrestrial temperatures (e.g., approximately 300.degree. K.) to cryogenic temperatures (e.g., below 50.degree. K.).
Pure silicon crystal is quite fragile, and could not survive the mechanical stresses involved in a rocket launch nor the stresses involved in extreme temperature cycling, unless it is properly mounted to withstand such stresses. Until the present invention, no effective technique was available for mounting an interferometric apparatus comprising a solid etalon made of pure silicon crystal in an interferometer designed to be launched by rocket into the upper atmosphere.